An ALD (Atomic Layer Deposition) method is known as a method for forming a film on a substrate such as a semiconductor wafer (hereinafter, referred to as “wafer”) or the like. The ALD method includes: adsorbing an atomic layer or a molecular layer of a raw material gas onto a surface of a wafer; supplying a reaction gas for oxidizing and reducing the raw material gas to produce reaction products; and depositing layers of the reaction products. The above operations are performed by accommodating the wafer into a processing chamber in which a vacuum atmosphere is formed and alternately and intermittently supplying the raw material gas and the reaction gas into the processing chamber by controlling opening/closing of a valve.
The raw material may be in a solid state or in a liquid state. By heating a raw material container accommodating the raw material, the raw material is vaporized to a raw material gas. A carrier gas is supplied into the raw material container. The raw material is supplied into the processing chamber by the carrier gas. The raw material gas is a mixture of the carrier gas and the vaporized raw material. An amount of the vaporized raw material (a flow rate of the raw material contained in the raw material gas) needs to be precisely controlled in order to control a thickness or a quality of a film to be deposited on the wafer.
The amount of the vaporized raw material in the raw material container varies depending on an amount of the raw material contained in the raw material container. When the raw material is in a solid state, the amount of the vaporized raw material varies depending on a change in a grain size, non-uniform distribution of the raw material in the raw material container or the like. Further, when the raw material is in a solid state, the temperature distribution in the raw material container may not be uniform. This is because convection hardly occurs in the raw material container even though a temperature in the raw material container is decreased by heat loss caused by the sublimation (considered as “vaporization” in the disclosure) of the raw material. Accordingly, the amount of the vaporized raw material may not be stable.
Japanese Patent Application Publication No. 2006-222133 (see paragraph [0036]) discloses a technique for measuring a raw material concentration in a mixed gas that is a mixture of a vaporized raw material (referred to as “raw material gas”) and a carrier gas by a densitometer and maintaining a total flow rate of the raw material gas, the carrier gas, and a dilution gas having the same component as that of the carrier gas at a constant level. In addition, Japanese Patent Application Publication No. 1993-305228 (see paragraphs [0015] to [0018]) discloses a technique for measuring a total mass flow rate of a non-evaporated gas in a system and controlling the total mass flow rate to a constant level when introducing a carrier gas into a liquid raw material evaporation unit and introducing a buffer gas into the system.
However, in the ALD method, the gas containing the vaporized raw material is intermittently supplied and the supply time thereof is considerably short. Therefore, it is not possible to precisely detect the raw material concentration by the densitometer described in Japanese Patent Application Publication No. 2006-222133. The supply timing of the gas containing the raw material is controlled by opening/closing of a valve installed in a supply line. In the case of measuring the mass flow rate as described in Japanese Patent Application Publication No. 1993-305228, the flow rate is not stable because the gas supply time is short. As a result, it is difficult to control the amount of the vaporized raw material to a set value.